Denial of service attack (DoS)

There are several tools that are commonly used by hackers to gain illegal access to computers and network to include backdoor. botnet, denial of service attack, phishing and spoofing. robot, zombie worm. Trojan. viruses, and worms.
What is a denial of service attack (DoS)?
What impact would a denial of service attack have on a major critical infrastructure such as mass transit?
Your response must be at least 200 words in length.